As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
In addition, recently, lithium ion batteries are commercially used as a power supply in home electronics such as laptop computers, mobile phones and the like. Furthermore, as interest in environmental problems is increasing, research into electric vehicles (EVs), hybrid electric vehicles (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is underway.
As a cathode of conventionally used lithium ion batteries, lithium cobalt oxides such as LiCoO2 having a layered structure are used. As an anode, graphite based materials are generally used.
Lithium cobalt oxides are currently widely used due to superior physical properties such as superior cycle characteristics as compared to LiNiO2 and LiMn2O4. To develop secondary batteries having high energy density, cathode active materials having large capacity are required. However, when operating voltage of lithium cobalt oxides are fixed unlike three component-based cathode active materials, it is substantially impossible to enlarge capacities of materials.
Accordingly, lithium cobalt oxides must be used under high voltage to develop secondary batteries having high energy density. However, approximately 50% or more of lithium ions are eliminated under high voltage operation, structures of lithium cobalt oxides collapse and, as such, lifespan characteristics are rapidly degraded.
To overcome this problem and to achieve high energy density, technologies substituting some cobalt with Al, Mg, B or the like, or treating surfaces of lithium cobalt oxides with a metal oxide such as Al2O3, Mg2O, TiO2 or the like are known.
However, when some cobalt is substituted with metals described above, there is still a problem such as degradation of lifespan characteristics. When a surface of a lithium cobalt oxide is coated with a metal oxide, specific capacity may be reduced due to addition of a coating material that does not directly participate in charge and discharge reaction, and a metal oxide with very low electrical conductivity mainly constitutes the coating material, which results in reduced conductivity. In addition, the coating process reduces active reaction area, thereby increasing interfacial resistance and deteriorating high-rate charge and discharge characteristics.
Therefore, there is an urgent need to develop technology for fundamentally addressing these problems and enhancing high voltage lifespan characteristics of a lithium cobalt oxide.